Some normal murine sera and serum and ascitic fluids from mice bearing transplantable tumors in ascites form are immunosupressive in vitro and in vivo. The goal of this project is the isolation and biological characterization of these factors. Methods involve characterization of the effect of these factors on mitogen induced lymphoblast transformation and the mixed lymphocyte reaction. The effect of these fluids on the primary and secondary in vitro antibody response is under investigation. Present emphasis is on establishing that target cell(s) upon which the inhibitor(s) act. We have focussed our attention on to activity in normal mouse serum. Correlations with activity in ascites fluids have been begun.